Over the last two decades, great strides have been made in the treatment of acute lymphoblastic leukemia (ALL). This progress has been paralleled by advances in diagnosis. In addition to morphology and cytochemistry, the diagnostic and prognostic importance of immunophenotypic and genetic features is becoming increasingly apparent. This article reviews the clinical and morphologic features, immunophenotypic classification, and karyotypic abnormalities of ALL, and discusses how these aspects relate to the diagnosis of ALL.